In the field of fluid flow, a weir is any dam or bulkhead over which a fluid may flow, or a bulkhead containing a notch through which a fluid may flow. Generally, a weir is useful for measuring the volume rate of fluid flow. Many types of weirs have been known and used in the past. Such weirs include sharp-crested and flat-crested, rectangular and V-notch, trapezoidal, broad-crested, circular and submerged. Various equations have been derived for the many types of weirs which exist. By measuring the physical characteristics of any particular weir and taking into account various flow impeding factors, it is possible to derive a volume rate of fluid flow equation for such weir. Once such an equation has been derived, it is then possible to use such weir to measure the volume rate of fluid flow by merely measuring the head of fluid over the inlet crest of such weir and placing the value thus obtained into the equation derived for such weir.
While prior art weirs have been found quite useful for measuring the volume rate of fluid flow, such weirs have generally not been suitable for providing and maintaining a selected steady-state volume rate of fluid flow from a variable depth fluid independent of the depth of the fluid. More particularly, prior art weirs have generally been of a fixed type. Accordingly, when the depth of the fluid would increase or decrease, the volume rate of fluid flow would undergo a corresponding increase or decrease. This can readily be understood since an increase or decrease in the depth of the fluid results in a corresponding increase or decrease of the head of fluid over the inlet crest of such fixed weirs. Therefore, while prior art weirs have been well-suited for measuring the volume rate of fluid flow, such weirs have not been suitable for applications which require maintaining a selected steady-state volume rate of fluid flow from a variable depth fluid independent of the depth of the fluid.
Various constant rate pumps have been developed to provide and maintain a selected steady-state volume rate of fluid flow from a variable depth liquid independent of the depth of the fluid. However, for many operations, constant rate pumps are simply not suitable. For instance, such pumps are not suitable for use in sewage treatment facilities since the fluids treated in such facilities are characterized by a presence of suspended solids and such fluids undergo variations in the rate of flow. Constant rate pumps cannot be adjusted to compensate for such variations in the flow rate of suspended solids composed in part of biological colonies called "floc". It is important that these colonies not be broken up. Most pumps, because of their high velocity impellers, will break up the biological floc and, therefore, are not desirable for this application in a sewage treatment facility. Finally, pumps are not suitable due to their high cost. Therefore, alternative means for providing and maintaining a steady-state volume rate of fluid flow for applications such as use in sewage treatment facilities have been sought.